A typical flow-control valve has a housing forming a flow passage having an intake port and an outlet port and a valve seat between the ports and having a lateral hollow part. An upstream valve body is engageable with the seat and forms an upstream restriction, and a downstream valve body forms a downstream restriction downstream of the upstream restriction. A control piston in the part defines therein an outer control compartment and an inner control compartment between the outer control compartment and the seat. This control piston is coupled to the downstream valve body for joint movement therewith. A high-pressure conduit connects the outer control compartment to the flow passage upstream of the upstream restriction, and a low-pressure conduit connects the inner control compartment to the flow passage between the upstream and downstream restrictions. A connector element or stem connected to the upstream valve body extends through the control piston and has an outer end projecting out of the housing.
In flow-control valves of this type, a flow passes through the upstream restriction that is defined by the position of a upstream valve body with respect to a valve seat and then the stream passes a downstream restriction that is defined by the outer valve body that is controlled by the difference in pressure, which also engages its own or the same valve seat. This difference in pressure is hereby given by the pressures that are active upstream of and downstream of the upstream restriction.
This difference in pressure is the control variable and acts upon a diaphragm that is connected to the downstream valve body. This diaphragm unit is displaceable so that the downstream valve body changes the downstream restriction in such a way that the difference in pressure across the upstream restriction remains constant and thus also the volume flow.
So that this displacement remains independent of differential pressure fluctuations via the downstream valve body, the pressure that is present downstream of the upstream restriction acts upon the inlet side as well as upon the downstream side that is directed toward the diaphragm unit of the downstream valve body, whereby both sides are separated from each other by a seal and the pressure is conveyed through passages of the diaphragm unit correspondingly to the downstream side. If the operating conditions in the heating or cooling system change, the volume flow be adapted, for which then the upstream restriction and thus the position of the upstream valve body is to be changed. This takes place, for example, manually or by motorized adjustment devices that are connected with the upstream valve body. Such a flow-control valve is known, for example, from DE 4,416,154.
To manage large volume flows with flow-control valves, the operating forces for changing the position of the upstream valve body increase so that manual actuation is more difficult or no longer possible at all. In motorized adjustment devices, correspondingly, very powerful adjustment mechanisms are required.